Benzaldehyde is widely used as an intermediate in the manufacture of dyestuff, flavoring chemicals (e.g. cinnamldehyde, amyl cinnamldehyde and benzyl benzoate), perfumery chemicals, in acriline dyes, pharmaceuticals and fine chemicals. It can be also used as a corrosion inhibitor and a solvent for polyester resins.
Benzaldehyde is produced by the hydrolysis of the corresponding side chain halogenated toluene compounds such as benzyl chloride at a temperature range of 100°–200° C. at normal or higher pressures in the presence of an excess hydrochloric acid (U.S. Pat. No. 4,229,379). In U.S. Pat. No. 4,450,298 a process is described for the vapor phase catalytic hydrolysis of benzyl chloride to benzaldehyde using a catalyst comprising activated carbon treated with sulphuric acid or impregnated with a metal chloride such as iron (III) chloride or a metal sulphate such as cupric sulphate. A major disadvantage of these processes is the generation of large amount of effluent. The benzaldehyde produced by these routes does not meet food grade specifications.
Vapor phase or liquid phase oxidation of toluene by air or O2 is environmentally benign and provided the desired selectivities to the market driven products. World patent WO 95/20560 discloses a liquid phase process for the manufacture of benzaldehyde by the oxidation of toluene in presence of oxygen in a temperature and pressure range of 120°–200° C. and 2–50 atm, respectively in the presence of a catalyst comprising cobalt or manganese as a metal ion and bromide as a promoter. 10% conversion of toluene and 45% selectivity to benzaldehyde is obtained. Borgaonkar et al, [I & EC Prod. Res. Dev., 23(3), 459 (1984)] have reported lower (10%) conversion of toluene and 90% yield of benzaldehyde by the use of cobalt acetate and either sodium bromide or paraldehyde as a promoter in presence of air.
A process for the vapor phase oxidation of toluene to benzaldehyde and benzoic acid using a catalyst containing a mixture of silver vanadate and lead vanadate or silver arsenate in presence of oxygen or ozone is described in U.S. Pat. No. 3,485,876. This catalyst system suppresses the formation of benzoic acid and degradation to carbon dioxide. According to U.S. Pat. No. 3,946,067, aromatic aldehydes such as benzaldehyde or substituted benzaldehydes were manufactured by the vapour-phase oxidation of aralkyl compounds, like toluene or substituted toluenes, in the presence of catalyst containing palladium metal and phosphoric acid at a temperature of less than 250° C. The aromatic aldehydes are produced in a single reaction step. The drawbacks of the process are that the conversion of toluene has to be kept very low (<4%) for obtaining high selectivity (>70%). Also, the process is not suitable as large amounts of carbon dioxide are formed due to high temperature used in the reaction. U.S. Pat. No. 4,390,728 describes a process in which the oxides of various metals (viz. Cu, Fe, Pb, Mo, U, and P) with promoter were used for the production of benzaldehyde by the oxidation of toluene. At a temperature of 475–550° C., a conversion of 35–50% and selectivity of 40–70% to benzaldehyde was obtained. U.S. Pat. No. 4,137,259 describes a process wherein silver vanadate and iron vanadate on silica is used as a catalyst at a temperature range 300°–500° C. in the presence of oxygen or ozone and steam. The conversion of toluene was found to be 21% and lower selectivity to aldehyde. A large amount of carbon oxide formation was observed in this process. U.S. Pat. No. 3,989,674, describes a process using Cu—Au silica catalyst system. In this process, a mixture of toluene, oxygen and helium in the molar ratio of 1:2:8 is passed over the catalyst at atmospheric pressure and temperature in the range of 230–390° C. The selectivity to benzaldehyde of 75–80% was obtained at a conversion level of only 15–30%. Another process for the production of benzaldehyde was reported by Ray et al. [Ind. J. Technol., 21(4), 137 (1983)] at a conversion of about 15% and selectivity of 70% with significant amount of CO2 formation. Hence, the major disadvantages of the above processes are the use of high temperatures and lower conversion of toluene. The formation of large amount of carbon dioxide ultimately will affect the overall yield and is also not environmentally clean. Thus, because of the above drawbacks these processes are not promising for the production of benzaldehyde by toluene oxidation.
Parteinheimer (J. Mol. Catal. 67, 35, 1991) has reported the use of HBr as a promoter in the cobalt catalyst system for the liquid phase oxidation of toluene. For this catalyst system, a very low yield for benzaldehyde (3%) and high yield of benzoic acid (91%) was obtained. Recently, in U.S. Pat. No. 6,495,726 a process is described for the production of benzaldehyde by liquid phase air oxidation of toluene using cobalt and manganese metal salts in the presence of zinc bromide as a promoter. The selectivity to benzaldehyde was reported to be 63% with toluene conversion of only 13%.